Revert "drm/i915/dp: Use auxch precharge value of 5 everywhere"

This reverts commit 092945e.

This commit prevents a DP screen from properly training the link.
Oddly enough it works, once the machine has been warm-booted with an
older kernel.

According to DP docs this _should_ have been the right precharge time.
Also, the commit that originally introduces this was just general snb
DP enabling and didn't mention any specific reason for this special
value. Whatever, trust the reporter that this makes things worse and
let's just revert it.

v2: Less spelling fail.

Cc: Adam Jackson <ajax@redhat.com>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Reported-by: "Wouter M. Koolen" <W.M.Koolen-Wijkstra@cwi.nl>
Buglink: https://lkml.org/lkml/2012/6/14/301
Cc: stable@vger.kernel.org (only for 3.4)
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_dp.c

@@ -371,7 +371,7 @@ intel_dp_aux_ch(struct intel_dp *intel_dp,
 	int recv_bytes;
 	uint32_t status;
 	uint32_t aux_clock_divider;
-	int try, precharge = 5;
+	int try, precharge;
 
 	intel_dp_check_edp(intel_dp);
 	/* The clock divider is based off the hrawclk,
@@ -391,6 +391,11 @@ intel_dp_aux_ch(struct intel_dp *intel_dp,
 	else
 		aux_clock_divider = intel_hrawclk(dev) / 2;
 
+	if (IS_GEN6(dev))
+		precharge = 3;
+	else
+		precharge = 5;
+
 	/* Try to wait for any previous AUX channel activity */
 	for (try = 0; try < 3; try++) {
 		status = I915_READ(ch_ctl);